Ethylene-vinyl alcohol copolymers (hereinafter often abbreviated to EVOHs), in particular, products of saponification of ethylene-vinyl acetate copolymers, are excellent in terms of various properties including gas barrier property and mechanical strength and are hence extensively used in various applications including films, sheets, containers, and fibers.
The saponification products are produced by copolymerizing ethylene and vinyl acetate, removing the unreacted vinyl acetate, and then saponifying the ethylene-vinyl acetate copolymers obtained.
Specifically, the polymerization for producing an ethylene-vinyl acetate copolymer is usually conducted using a high-pressure polymerization tank equipped with a jacket and a reflux condenser, and ethylene and vinyl acetate are introduced as starting materials into an alcohol solvent to conduct the polymerization. After completion of the polymerization, the unreacted vinyl acetate is removed to obtain an ethylene-vinyl acetate copolymer. Usually, the unreacted vinyl acetate is removed to 100 ppm or less. Thereafter, the copolymer is further saponified with an alkali catalyst into an EVOH, which is usually extruded in the form of strands into a coagulating bath and cut into pellets.
EVOHs generally have a tendency that the higher the ethylene content, the better the melt moldability and stretch formability and that the lower the ethylene content, the better the gas barrier properties. It is, however, difficult to attain both melt moldability or stretch formability and gas barrier properties. In order to obtain an EVOH which combines stretch formability and gas barrier properties, a process has been proposed in which copolymerization of ethylene and vinyl acetate is conducted under two or more different sets of conditions so that, for example, the ethylene pressure and/or the temperature is changed stepwise (see, for example, Patent Document 1).